Many textile fabrics, and in particular those made wholly or partly from cellulosic fibers, have a tendency to shrink undesirably as a result of becoming wet or undergoing conventional laundering processes. To obviate undesirable shrinking, many such fabrics are customarily treated using a compressive or compaction shrinkage process, in order to pre-shrink the fabrics and increase their stability. Examples of compressive shrinkage processes are described in U.S. Pat. No. 2,146,694 to Wrigley, et al. and U.S. Pat. No. 3,469,292 to Hojyo, U.S. Pat. No. 4,156,955 to Joy, and U.S. Pat. No. 4,446,606 to Lawrence et al, the disclosures of which are incorporated herein by reference. Also, a popular compressive shrinkage process is known by the tradename SANFORIZE.
In compressive shrinkage processes, a fabric web is typically laid out over the working face of a thick endless rubber blanket so that it is free of folds or wrinkles. The rubber blanket is positioned on a plurality of rotatable rolls which support the blanket along its 10 bearing surface, and the blanket is typically conveyed along an endless path by way of a driven cylinder which contacts the outer blanket surface. In this way, the fabric web placed on the outer surface of the blanket is caused to be carried through a number of processing stations.
First, the fabric is typically moistened, then it is compressed along with the blanket between a roll and a heated cylinder or shoe. As the fabric and blanket pass between the nip (i.e., the point of contact between the two contiguous elements) and the blanket is compressed, adjacent portions of the outer surface of the blanket are caused to be extended. As the blanket and fabric leave the roll, the blanket contracts, and the fabric is forced to follow suit. As a result, the yarns in the warp direction are caused to shorten, and the filling yarns are pushed upwardly, thereby mechanically shrinking the fabric. The fabric is then fed to a dryer, where it is dried in its preshrunk condition.
Because the rubber blanket is endless, a web of fabric can be processed in a continuous manner. However, the surface of the rubber blanket must be cooled following contact with the heated cylinder before it again contacts the fabric web. Such cooling is generally performed by applying water to the blanket as it travels between the point of web removal and the point of untreated fabric web lay-down. Because too much moisture on the blanket can interfere with proper fabric conditioning, it is generally necessary that the amount of water on the blanket working surface be closely controlled. Generally this is performed by water removal rolls, which squeegee the excess water from the cooled blanket. Because it is important that the blanket stay properly lubricated, water is often added to the bearing surface of the blanket at various positions throughout the process, e.g., before the point of fabric lay-down and following contact of the blanket with the heated cylinder.
As should be apparent, the rubber blankets are exposed to great stresses during the compression shrinkage process as a result of the repeated heating and cooling, the tensions at which the blanket must be run on the machine, the compression forces endured by going through the nip, and the repeated wetting operations. Under these conditions, the working surface of the blanket slowly oxidizes. This results in an increase in hardness and a decrease in wettability. In addition, finishes present on the fabric surface are often transferred to the rubber surface. Over a relatively short time this finish tends to form a glaze on the rubber surface, further decreasing the wettability and friction characteristics of the surface. As will be readily appreciated by those of ordinary skill in the art, the reduction in frictional characteristics on the web-contacting surface of the blanket reduces its effectiveness in gripping the fabric web. As a result, the surface characteristics of the blanket must be modified to restore its frictional characteristics in order that it can continue to properly and uniformly process fabrics.
For example, in commercial applications, once the blanket hardness has been found to deviate upwardly or downwardly about 12% from its original level, blanket manufacturers recommend that the blanket be ground to remove the dead rubber on its surface. In this way, the surface of the blanket is prevented from becoming too slick or from losing its ability to grab hold of the fabric being treated. Such grinding is usually performed by stopping the machine and backing the rubber blanket up against a rotatable roll covered with abrasive material (e.g., grinding cloth or sandpaper), which grinds the working face of the rubber blanket until the dead rubber area has been removed.
Typically the grinding process requires the removal of about a sixteenth of an inch of the blanket surface with each grinding. Because, for example, a blanket which begins at 3 inches thick usually must remain at least two inches thick to work effectively, the number of grindings is thus very limited. As a result, the life of the rubber blanket used in these types of apparatus can be undesirably short.
It can also be appreciated that intermittent grinding of the blanket produces a surface that is variable over time, resulting in a greater amount of variability in compressive efficiency, and greater variability in the shrinkage characteristics of the final product. As the overall pre-shrinkage may need to be increased to avoid producing out-of-specification goods, the fabric yield will be less.
In addition, small cuts and nicks in the blanket can form and grow over time due to oxidation and the constant stretching and releasing of the blanket rubber surface. When the blanket is ground, additional blanket thickness must be sacrificed in order to insure that all cracks are removed. This contributes to a shorter blanket life.
During grinding of the blanket, production is halted, as the blanket must be ground dry to avoid premature decomposition or destruction of the grinding cloth or sandpaper. In addition, a considerable amount of rubber debris is formed due to the conventional grinding process. A heavy dusting of talc is typically applied during the grinding process, to reduce the friction and heat generated and to keep the blanket from becoming too sticky during the grinding operation. This talc and surplus rubber material must be cleaned from the blanket to prevent them from collecting on fabrics or materials being processed after the grinding operation.
In addition, blankets typically require frequent cleaning to remove the build-up of baked-on fabric finishes, oils, and the like. Again, production must be halted so that the blanket may be cooled, and detergents applied. However, if such finishes and oils are not removed on a timely basis, they can adversely affect the process performance as well as contribute to the decomposition of the rubber blanket.
The requirements of frequent cleaning and grinding prevent the rubber blanket machine from operating in-line with modern webprocessing equipment, which generally operate continuously, and which cannot economically be stopped to accommodate belt cleaning and grinding. A typical blanket grinding operation takes about 8 hours to perform, which is significant lost time from a fabric producer's perspective. Therefore, the grinding operation is recognized as being a significant source of machine downtime.
One attempt to increase the lifespan of blankets in compressive shrinkage apparatus is described in U.S. Pat. No. 5,791,029 to Maker, the disclosure of which is incorporated herein by reference. The '029 patent describes a rubber blanket construction having a bearing face which is beveled. The patentee describes that this construction reduces the tendency of the edges of the blanket to curve upwardly when the blanket is tensioned to perform a grinding operation and reduces the tendency of the edges to crack. While this method may reduce the tendency of the blanket to crack, it does not overcome the need for frequent blanket cleaning and grinding.